1. Field of the Invention
The disclosures herein generally relate to memory devices, and particularly relate to a nonvolatile memory device.
2. Description of the Related Art
There are various types of nonvolatile memory devices. Conventionally, a nonvolatile memory cell requires a special structure such as a floating gate or a special material such as a ferroelectric material for the purpose of achieving nonvolatile data retention. There is a new type of nonvolatile memory device known as a PermSRAM, which uses an MIS (metal-insulating film-semiconductor) transistor as a nonvolatile memory cell. In this type of nonvolatile memory device, data is written to an MIS transistor by deliberately causing electrons to be trapped in an insulator layer or sidewalls through the hot-carrier effect.
Some types of nonvolatile memory devices employ a memory cell through which an electric current flows during a data read operation. In such nonvolatile memory devices, a current sense circuit compares a current flowing through an accessed memory cell with a current flowing through a reference cell. A data value read from the accessed memory cell is determined based on the result of the comparison. A cell current flowing through an accessed memory cell that stores “0” may be smaller than a reference current flowing through the reference cell. A cell current flowing through an accessed memory cell that stores “1” may be larger than the reference current.
A large cell current flows through each of the accessed memory cells that stores “1”. The number of memory cells accessed at a time is equal to the number of bits for data input/output. When the data bit width is 32, for example, the total amount of currents flowing through the accessed memory cells can reach 32 times the amount of the large cell current. Such a large current consumption may not be desirable when considering a demand for reduced power consumption.